The present invention relates to the field liquid crystal optical devices.
The present invention was made at the Laboratoire de Physique des Solides (Solid-State Physics Laboratory) of the Universite de Paris Sud (University of Paris South), which laboratory is associated with the CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE (NATIONAL CENTER FOR SCIENTIFIC RESEARCH) number 04 0002.
Much research worked,has been conducted for at least the last fifteen years or so on liquid crystals.
Various results of the research work carried out at the Laboratoire de Physique des Solides de l'Universite Paris Sud are described in the French Patent Application filed on Apr. 28, 1982 under the No. 82 07309 and published under the No. 2 526 177, the French Patent Application filed on Oct. 23, 1984 under the No. 84 16192 and published under the No. 2 572 210, the French Patent Application filed on Jun. 18, 1985 under the No. 85 09224 and published under the No. 2 587 506, the French Patent Application filed on May 14, 1986 under the No. 86 06916 and published under the No. 2 598 827 or again the French Patent Application filed on Dec. 17, 1987 under the No. 87 17660 and published under the No. 2 624 985.
Moreover, the work relating to liquid crystals has given rise to numerous publications.
The present invention more precisely relates to the liquid crystal optical devices which are called bistable, that is to say devices in which the molecules of the liquid crystals can alternately occupy two stable states, under the effect of external control. Such bistable optical devices are suitable in particular for producing multiplexed displays.
Various liquid crystal bistable optical devices have already been proposed.
The document Applied Physic [sic] Letters 40 (12) 1007 (1982) J. Cheng et al. describes, for example, a nematic liquid crystal device having two states exhibiting bulk stability switched by an external command electric field. The process described in this document has not given rise to a practical application. It has a very slow commutation time and generally exhibits numerous textural defects.
The document Applied Physic [sic] Letters 36, 899 (1980), N. A. Clark et al. describes another bistable optical device using liquid crystals called Smectic C* ferroelectric, and degenerate surface anchorings. The process described in this document has the advantage of a very short switching time and has given rise to practical applications. It is not however completely satisfactory.
In particular, in practice, it is frequently noticed that instead of a display arrangement which is bistable between two symmetric states, display arrangements which are monostable on twisted textures whose contrast is poor and which cannot be multiplexed are obtained. This phenomenon seems to be due to the fact that the electrode/liquid crystal interface is polar.
The document Applied Physic [sic] Letters Dec. 11, 1989, R. Barberi, M. Boix and G. Durand describes another bistable optical device in which the bistability is induced by a controlled toughening treatment on at least one of the transparent electrodes and the switching is operated by application of an external electric field parallel to the electrodes. According to this document, the toughening treatment may be obtained, for example, by oblique evaporation of SiO. This Applied Physic [sic] Letter [sic] document is to be linked with the aforementioned French Patent Application No. 87 17660. The process described in the document Applied Physics Letters Dec. 11, 1989 seems promising. The specialists have, however, always hitherto considered that this process has the major drawback of being sensitive only to an electric field parallel to the transparent plates of the device and of being completely insensitive to an electric field perpendicular to the plates.
Another type of nematic bistable display using the bistability of surface orientation states, in which the switching controlled by electric pulses of defined polarity is based on the use of chiral ions, is described in the French Patent Application filed on Jan. 30, 1990 under the No. 90 01066.